It is well known that nickel based superalloys are extensively employed in high performance environments. Such alloys have been used extensively in jet engines, in land based gas turbines and other machinery where they must retain high strength and other desirable physical properties at elevated temperatures of 1000.degree. F. or more.
Many of these alloys contain a .gamma.' precipitate in varying volume percentages. The .gamma.' precipitate contributes to the high performance properties of such alloys at their elevated use temperatures.
More detailed characteristics of the phase chemistry of .gamma.' are given in "Phase Chemistries in Precipitation-Strengthening Superalloy" by E. L. Hall, Y. M. Kouh, and K. M. Chang [Proceedings of 41st Annual Meeting of Electron Microscopy Society of America, Aug. 1983 (p. 248)].
The following U.S. patents disclose various nickel-base alloy compositions: U.S. Pat. Nos. 2,570,193; 2,621,122; 3,046,108; 3,061,426; 3,151,981; 3,166,412; 3,322,534; 3,343,950; 3,575,734; 3,576,861; 4,207,098 and 4,336,312. The aforementioned patents are representative of the many alloying developments reported to date in which many of the same elements are combined to achieve distinctly different functional relationships between the elements such that phases providing the alloy system with different physical and mechanical characteristics are formed. Nevertheless, despite the large amount of data available concerning the nickel-based alloys, it is still not possible for workers in the art to predict with any significant degree of accuracy the physical and mechanical properties that will be displayed by certain concentrations of known elements used in combination to form such alloys even though such combination may fall within broad, generalized teachings in the art, particularly when the alloys are processed using heat treatments different from those previously employed.
A problem which has been recognized to a greater and greater degree with many such nickel based superalloys is that they are subject to formation of cracks or incipient cracks, either in fabrication or in use, and that the cracks can actually propagate or grow while under stress as during use of the alloys in such structures as gas turbines and jet engines. The propagation or enlargement of cracks can lead to part fracture or other failure. The consequence of the failure of the moving mechanical part due to crack formation and propagation is well understood. In jet engines it can be particularly hazardous.
U.S. Pat. No. 4,685,977 entitled "Fatigue-Resistant Nickel-Base Superalloys and Method" is assigned to the same assignee as the subject application. It discloses an alloy having a superior resistance to fatigue crack propagation based on alloy chemistry, .gamma.' precipitate content and grain structure. A method of alloy preparation is also taught.
What has been poorly understood until recent studies were conducted was that the formation and the propagation of cracks in structures formed of superalloys is not a monolithic phenomena in which all cracks are formed and propagated by the same mechanism and at the same rate and according to the same criteria. By contrast the complexity of the crack generation and propagation and of the crack phenomena generally and the interdependence of such propagation with the manner in which stress is applied is a subject on which important new information has been gathered in recent years. The variability from alloy to alloy of the effect of the period during which stress is applied to a member to develop or propagate a crack, the intensity of the stress applied, the rate of application and of removal of stress to and from the member and the schedule of this application was not well understood in the industry until a study was conducted under contract to the National Aeronautics and Space Administration. This study is reported in a technical report identified as NASA CR-165123 issued from the National Aeronautics and Space Administration, NASA Lewis Research Center, Contract NAS3- 21379.
A principal finding of the NASA sponsored study was that the rate of propagation based on fatigue phenomena or in other words, the rate of fatigue crack propagation (FCP), was not uniform for all stresses applied nor to all manners of applications of stress. More importantly, the finding was that fatigue crack propagation actually varied with the frequency of the application of stress to the member where the stress was applied in a manner to enlarge the crack. More surprising still, was the magnitude of the finding from the NASA sponsored study that the application of stress of lower frequencies rather than at the higher frequencies previously employed in studies, actually increased the rate of crack propagation. In other words, the NASA study verified that there was a time dependence in fatigue crack propagation. Further, the time dependence of fatigue crack propagation was found to depend not on frequency alone but on the time during which the member was held under stress or a so-called hold-time.
Following the documentation of this unusual degree of increased fatigue crack propagation at lower stress frequencies there was some belief in the industry that this newly discovered phenomena represented an ultimate limitation on the ability of the nickel based superalloys to be employed in the stress bearing parts of the turbines and aircraft engines and that all design effort had to be made to design around this problem.
However, it has been discovered that it is feasible to construct parts of nickel based superalloys for use at high stress in turbines and aircraft engines with greatly reduced crack propagation rates and with good high temperature strength.
It is known that the most demanding sets of properties for superalloys are those which are needed in connection with jet engine construction. Of the sets of properties which are needed those which are needed for the moving parts of the engine are usually greater than those needed for static parts, although the sets of needed properties are different for the different components of an engine.
Because some sets of properties are not attainable in cast alloy materials, resort is sometimes had to the preparation of parts by powder metallurgy techniques. However, one of the limitations which attends the use of powder metallurgy techniques in preparing moving parts for jet engines is that of the purity of the powder. If the powder contains impurities such as a speck of ceramic or oxide the place where that speck occurs in the moving part becomes a latent weak spot where a crack may initiate. Such a weak spot is in essence a latent crack. The possible presence of such latent cracks makes the problems of reducing and inhibiting the crack propagation rate all the more important. I have found that it is possible to inhibit crack propagation both by the control of the composition of alloys and by the methods of preparation of such metal alloys.
Pursuant to the present invention, a superalloy which can be prepared by powder metallurgy techniques is provided. Also, a method for processing this superalloy to produce materials with a superior set of combination of properties for use in advanced engine disk application is provided. The properties which are conventionally needed for materials used in disk applications include high tensile strength and high stress rupture strength. In addition, the alloy of the subject invention exhibits a desirable property of resisting time dependent crack growth propagation. Such ability to resist crack growth is essential for the component low cycle fatigue (LCF) life.
As alloy products for use in turbines and jet engines have developed it has become apparent that different sets of properties are needed for parts which are employed in different parts of the engine or turbine. For jet engines the material requirements of more advanced aircraft engines continue to become more strict as the performance requirements of the aircraft engines are increased. The different requirements are evidenced, for example, by the fact that many blade alloys display very good high temperature properties in the cast form. However, the direct conversion of cast blade alloys into disk alloys is very unlikely because blade alloys display inadequate strength at intermediate temperatures. Further, the blade alloys have been found very difficult to forge and forging has been found desirable in the fabrication of disks from disk alloys. Moreover, the crack growth resistance of disk alloys has not been evaluated. Accordingly, to achieve increased engine efficiency and greater performance constant demands are made for improvements in the strength and temperature capability of disk alloys as a special group of alloys for use in aircraft engines.
Accordingly, what was sought in undertaking the work which lead to the present invention was the development of a disk alloy having a low or minimum time dependence of fatigue crack propagation and moreover a high resistance to fatigue cracking. In addition what was sought was a balance of properties and particularly of tensile, creep and fatigue properties. Further, what was sought was an enhancement of established alloy systems relative to inhibition of crack growth phenomena.
The development of the superalloy compositions and methods of their processing of this invention focuses on the fatigue property and addresses in particular the time dependence of crack growth.
Crack growth, i.e., the crack propagation rate, in high-strength alloy bodies is known to depend upon the applied stress (.alpha.) as well as the crack length (a). These two factors are combined by fracture mechanics to form one single crack growth driving force; namely, stress intensity factor K, which is proportional to .alpha..sqroot.a. Under the fatigue condition, the stress intensity in a fatigue cycle may consist of two components, cyclic and static. The former represents the maximum variation of cyclic stress intensity (.DELTA.K), i.e., the difference between K.sub.max and K.sub.min. At moderate temperatures, crack growth is determined primarily by the cyclic stress intensity (.DELTA.K) until the static fracture toughness KIC is reached. Crack growth rate is expressed mathematically as da/dN .varies.(.DELTA.K).sup.n. N represents the number of cycles and n is material dependent. The cyclic frequency and the shape of the waveform are the important parameters determining the crack growth rate. For a given cyclic stress intensity, a slower cyclic frequency can result in a faster crack growth rate. This undesirable time-dependent behavior of fatigue crack propagation can occur in most existing high strength superalloys. To add to the complexity of this time-dependence phenomenon, when the temperature is increased above some point, the crack can grow under static stress of some intensity K without any cyclic component being applied (i.e. .DELTA.K=0). The design objective is to make the value of da/dN as small and as free of time-dependency as possible. Components of stress intensity can interact with each other in some temperature range such that crack growth becomes a function of both cyclic and static stress intensities, i.e., both .DELTA.K and K.